The present invention relates to acoustic antennas, namely devices used for the transmission, by electrical signals, of acoustic, sonic or ultrasonic waves in water. Such antennas are used especially in sonar. The invention can be used, in particular, to send high acoustic power, or even very high acoustic power, with such an antenna.
It is known, in the field of signal processing and especially in sonar, that the greater the duration T of the pulses sent, the greater will be the processing gain which is proportional to the product BT (B: frequency band), and hence the greater the increase in detection performance.
There are known high-frequency transducers, typically used for transmission frequencies of over 50 KHz, constituted by a stack of layers known as “front” layer(s) (thin matching layers and/or tight-sealing membrane), a layer of active material (electrical/acoustic transduction) and layer(s) known as backing layer(s).
The phenomena of heating in the layer of active material, due to dielectric and mechanical losses, restrict the transmission peak power when the duration of the pulse is increased. Thus, for a material consisting of piezoelectric ceramics, the typical functioning of a transducer roughly follows the profile shown in FIG. 1.
This limiting of the level of allowable power for the long pulses is related to the use of materials having low heat conductivity for the matching thin layers, the backing as well as the tight-sealing membrane. Indeed, in the prior art, these elements are made out of materials comprising an elastomer matrix (such as rubbers, polyurethanes and silicones) or resin, especially epoxide, giving rise to inefficient discharge of the heat generated by the transducer into the carrier structure or into seawater.
There are known heat-conductive materials that take the form of foams. Reference may be made especially to the metal foams made of aluminum, nickel, nickel-chrome, copper or steel, as well as to non-metal foams made of carbon or silicon carbide. These foams have heat conductivity about 20 times greater than that of the charged epoxy resin type of composite materials used as matching or backing materials in the high-frequency transducers corresponding to the prior art. It is 50 times greater than that of rubbers constituting the impervious membranes used in these transducers.
From the German patent 19 623 035 filed by the firm STN Atlas, there is a known low-frequency transducer whose horn and/or rear mass are constituted by an expanded metal whose density is adjusted to obtain a determined resonance frequency.
To this end, the horn and/or the rear mass are obtained by molding the basic metal with an appropriate dose of foaming agent. However, this method of manufacture is difficult to implement and control, and this is a serious drawback.